Study of Wγγ tri-boson production in proton-proton collisions with the ATLAS detector.

From 2015 to 2018, the Large Hadron Collider (LHC) collided protons at an unprecedented centre of mass energy of √s = 13 TeV. The ATLAS detector recorded an integrated luminosity of 139 fb−1 of these collisions hence offering an unprecedented opportunity to test the Standard Model (SM) of particle physics by measuring predicted but yet unobserved rare processes. The tri-boson Wγγ production is one of these unobserved processes. Its sensitivity to the electroweak trilinear and quartic gauge couplings make it a great probe of new physics phenomena as Beyond Standard Model processes could affect the effective strength of these couplings. This thesis presents the study of the Wγγ process. Backgrounds are estimated from a combination of Monte Carlo (MC) simulations and data-driven techniques. The dominant source of background to the search for Wγγ production are jets being misidentified as photons. The advanced data-driven technique used to estimate this background is presented in details. Finally thorough examination of the systematic uncertainties affecting the measurement of the Wγγ production cross-section are presented. Taking into account all statistical and systematic uncertainties, the expected statistical significance of the measurement is of 5.1 σ for the differential measurement and 7.8 σ for the total cross-section.